Default show_only_kubernetes_context to false

`kubernetes_context.sh` is called with positional args as such:

```
kubernetes_context.sh $eks_hide_arn $eks_extract_account $hide_kubernetes_user $show_only_kubernetes_context $show_kubernetes_context_label
```

When `show_only_kubernetes_context` defaults to "", the positional arg $5 for  `show_kubernetes_context_label`,  becomes arg $4 for `show_only_kubernetes_context`. So the default value for `show_only_kubernetes_context` needs to be a non-zero string.

scripts/dracula.sh

@@ -10,7 +10,7 @@ main() {
   show_krbtgt_label=$(get_tmux_option "@dracula-krbtgt-context-label" "")
   krbtgt_principal=$(get_tmux_option "@dracula-krbtgt-principal" "")
   show_kubernetes_context_label=$(get_tmux_option "@dracula-kubernetes-context-label" "")
-  show_only_kubernetes_context=$(get_tmux_option "@dracula-show-only-kubernetes-context" "")
+  show_only_kubernetes_context=$(get_tmux_option "@dracula-show-only-kubernetes-context" false)
   eks_hide_arn=$(get_tmux_option "@dracula-kubernetes-eks-hide-arn" false)
   eks_extract_account=$(get_tmux_option "@dracula-kubernetes-eks-extract-account" false)
   hide_kubernetes_user=$(get_tmux_option "@dracula-kubernetes-hide-user" false)